Heart valve replacement is the second most common cardiac surgery in the United States, and aortic valve sclerosis, an indicator of calcification, occurs in >25% of aged individuals. Recent findings have established common molecular interactions in valve development and adult disease. However, a cellular basis for valve regeneration or repair has not yet been identified. Preliminary studies demonstrate that the bHLH transcription factor Twist1 promotes proliferation and migration while inhibiting differentiation of valve progenitor cells. During normal development, Twist1 expression is downregulated during valve remodeling, and expression is undetectable in normal adult valves. However, Twist1 expression is increased in diseased human valves in regions with increased cell proliferation and ECM disorganization, supporting a role for Twist1 in adult valve pathogenesis and potentially repair. We hypothesize that Twist1 promotes valve progenitor cell proliferation and inhibits differentiation, thereby maintaining the progenitor population, during embryonic development and postnatal valve pathogenesis. The proposed studies will dissect the cellular and molecular mechanisms of valve progenitor generation and maintenance in heart valve development and disease. The aims are 1) Determine if Twist1 homo- and hetero-dimers have differential functions in valve progenitor cell proliferation, migration and differentiation. 2) Determine if Twist1 expression is sufficient to generate valve progenitors by inducing EMT and cell proliferation in late stages of embryonic valve development and in mature adult valves. 3) Define the pathology associated with Twist1 induction in human valve disease and determine if Twist1 expression prevents valve disease progression in mice. The long-term goals of these studies are the definition of critical regulatory pathways in heart valve cell lineage development and the identification of potential regenerative mechanisms with therapeutic applications in valve disease.